עקרונות תכנות מונחה עצמים תרגול 21: Generics

Outline Generic classes Generics & Inheritance Wild Cards characters Case study: Generic Graph

Generic Classes – Motivation List v = new ArrayList(); v.add("test"); Integer i = (Integer)v.get(0); => // runtime error List<String> v = new ArrayList<String>(); v.add("test"); Integer i = (Integer)v.get(0); => // compilation error

class name<T1, T2, ..., Tn> { /* ... */ } Generic Classes A class is generic if it declares one or more type variables. A generic class is defined with the following format: class name<T1, T2, ..., Tn> { /* ... */ } The type parameter section, delimited by angle brackets (<>), follows the class name. It specifies the type parameters (also called type variables) T1,T2, ..., and Tn. The type variables can be used as: Method parameters. Return value. Local variables.

Generic Classes – Example public interface List<E> { void add(E x); Iterator<E> iterator(); } public interface Iterator<E> { E next(); boolean hasNext(); public class ArrayList<E> implements List<E> { private E[] _data; ...

Generic Classes vs Concrete Classes Generic Class declaration defines a set of parameterized types, one for each possible invocation of the type parameter section. All compiles to the same .class file. All uses the same .class file at runtime.

Generic Classes VS Concrete Classes (Example) public class ArrayList<E> implements List<E> { private E[] _data; ... } List<String> v; v = new ArrayList<String>(); Generic class Compile to ArrayList.class Concrete class Uses ArrayList.class

Generics & Inheritance

Assignments roles: If a type A is of a sub-class of type B we will mark it as: 𝐴≼𝐵 We can assign (runtime) type A to a (compile time) variable B iff 𝐴≼𝐵

Using wild cards – Motivation 𝐷𝑜𝑔≼𝐴𝑛𝑖𝑚𝑎𝑙 Animal animal; animal = new Dog(); Cat cat = animal; // // compilation error!!! 𝐴𝑛𝑖𝑚𝑎𝑙≼𝐶𝑎𝑡 Vector<Animal> animals; Vector<Cat> cats = new Vector<Cat>(); animals = cats; animals.add(new Dog()); Cat cat = cats.elementAt(0); // // compilation error!!! 𝑉𝑒𝑐𝑡𝑜𝑟<𝐶𝑎𝑡>≼𝑉𝑒𝑐𝑡𝑜𝑟<𝐴𝑛𝑖𝑚𝑎𝑙> 𝐷𝑜𝑔≼𝐴𝑛𝑖𝑚𝑎𝑙

Using wild cards 𝐴≼𝐵 ↛𝐺<𝐴>≼𝐺<𝐵> 𝐴≼𝐵 𝑖𝑚𝑝𝑙𝑖𝑒𝑠: We can read type B from A. We can write type A to B. We will use wild card (?) to represent an unknown type.

Using wild cards ? extends A – a variable type which can only be read from. We can write only null to it. ? super B – a variable type which can only be written to. We can read only Object from it. 𝐴≼𝐵→𝐺<𝐴> ≼𝐺<? 𝑒𝑥𝑡𝑒𝑛𝑑𝑠 𝐵>. 𝐴≼𝐵→𝐺<𝐵> ≼𝐺<?𝑠𝑢𝑝𝑒𝑟 𝐴>.

Using wild cards – example: public static void makeNoise (Vector<Animal> animals) { for (Animal animal:animals) { animal.say(); } { Vector<Cat> cats = new Vector<Cat>(); makeNoise(cats); => Correction: public static void makeNoise (Vector<? extends Animal> animals) // compilation error!!!

Using wild cards – example: Vector<? extends Animal> animals = null; Vector<Cat> cats = new Vector<Cat>(); animals = cats; animals.add(new Dog()); Cat cat = cats.elementAt(0); => // compilation error!!! Correction: animals.add(null); - The only one that allows.

Using wild cards – examples: Vector<? extends Animal> animals = new Vector<Cat>(); Vector<? extends Cat> cats = new Vector<Animal>(); Vector<? super Animal> animals2 = new Vector<Cat>(); Vector<? super Cat> cats2 = new Vector<Animal>(); animals.add(new Cat()); cats.add(new Cat()); animals.add(null); Cat cat = animals.elementAt(0); Animal animal = cats.elementAt(0); Cat cat2 = animals2.elementAt(0); Animal animal2 = animals2.elementAt(0); Object o = animals2.elementAt(0); // compilation error // compilation error // compilation error // compilation error // compilation error // compilation error // compilation error

Question from 2013 test. נתון שהמחלקה Bיורשת מהמחלקה A, והמחלקה G היא גנרית עם פרמטר יחיד. מה צריך להיות הפרמטר X כדי שההשמה הבאה תהיה תקפה ע"פ כללי הטיפוסים של ג'אווה? G<X> g = new G<B>(); א. A ב. extends A ? ג. super A ? ד. super B ? ה. תשובות ב, ד נכונות ו. תשובות א ,ג נכונות

Question from 2013 test. נתון שהמחלקה Bיורשת מהמחלקה A, והמחלקה G היא גנרית עם פרמטר יחיד. מה צריך להיות הפרמטר X כדי שההשמה הבאה תהיה תקפה ע"פ כללי הטיפוסים של ג'אווה? G<X> g = new G<B>(); א. A ב. extends A ? ג. super A ? ד. super B ? ה. תשובות ב, ד נכונות ו. תשובות א ,ג נכונות

Case study: Generic Graph

Graph

Graphs can model The internet Maps Social networks …

Graph Representation

The vertex class

The Graph Class

The Graph Class

Vertex class - code

Graph class - code

DFS Tour

DFS Tour

Execution example

The Weighted interface

Weight query

Usage example

Usage example (cont.)

Exception handling

Exception handling

Summary Generic class ( List<E> ) 𝐴≼𝐵 ↛𝐺<𝐴>≼𝐺<𝐵> ? extends A ? super A

Summary Generic class ( List<E> ) 𝐴≼𝐵 ↛𝐺<𝐴>≼𝐺<𝐵> ? extends A ? super A